Chromatin Modification Flashcards
In interphase, chromosomes are spread out in the nucleus. The spread out bits are called
Euchromatin
Some parts of chromosomes are condensed in interphase. The condensed bits are called
Heterochromatin
DNA in heterochromatin
is inaccessible for transcription
What is X-chromosome inactivation?
In female mammals, only one X chromosome is active the other is always condensed and inactive.
Expressed genes have a more open structure than others, shown by …
greater sensitivity to DNase
In the expressed state the only packaging is into nucleosomes
B-Globin genes are transcribed only in..
erythroid cells.
Where would the B-Globin gene be most sensitive to DNase?
Erythroid cells.
Opening up of chromatin involves what?
1) Histone modification, especially histone acetylases.
2) Chromatin remodelling complexes
What is histone acetylation?
Addition of acetyl groups to lysine residues. Acetylation removes a positive charge from the histone.
Acetylation reduces charge, changing interactions of histones with DNA and each other - LOOSENS NUCLEOSOMES.
What is the chromatin remodelling complex?
INCREASES ACCESSIBILITY OF DNA.
Active process, requires energy from ATP hydrolysis.
Enzymatically allows access of DBP by partly unwrapping or removing nucleosomes.
What are the three types of RNA polymerase:
RNA polymerase I: transcribes ribosomal RNA genes
RNA polymerase II: transcribes genes coding for proteins
RNA polymerase III: transcribes genes coding for small RNAS
All forms of eukaryotic RNA polymerase have subunits that correspond to the
a, beta and beta prime subunits of bacterial RNA polymerase. The structures of bacterial and eukaryotic RNA polymerases overlap.
Purification of eukaryotic RNA polymerases gives…
core enzymes which do not initiate transcription specifically. A transcription complex of at least 50 polypeptides is required for transcription in vivo.
What does specific initiation of transcription require?
- Enzymes that alter chromatin structure
- Specific activator proteins
- Mediator
- Basal or general transcription factors
- RNA polymerase core enzyme
What are some examples of basal factors for transcription by RNA polymerase II
TFIIA, TFIIB
What does the TATA box do?
directs RNA polymerase to the correct starting point.
The TATA box is about 25 bp upstream of the transcription start point.
To initiate transcription, TATA box binding protein binds to TATA box. What does this binding cause?
Put a wedge into the minor groove of the DNA molecule, twisting the DNA so it bends at an angle of about 70.
TBP forms part of what complex?
TFIID.
TFIID contains 8 other polypeptide subunits (TAFS) for TATA box associated factors.
Many genes don’t have a TATA box, some have one or more other sequences that define where transcription starts.
TRUE OR FALSE?
TRUE.
These sequences bind to TFIBB or other subunits of TFIID. However, TBP is still required for transcription and bending of DNA.
What are housekeeping proteins
Proteins that are required in all or most cells in relatively small amounts.
What is unusual about transcription of housekeeping genes?
Transcription of these genes does not start at a specific base: DISPERSED initiation.
What happens in preparation of transcription?
Transcription activator proteins bind to specific sequences in DNA, displacing nucleosomes.
To increase accessibility, histones are acetylated and chromatin is remodelled.
When gene is about to be transcribed, additional activator proteins bind to additional specific sequences in its DNA.
What happens in assembly of transcription complex?
a) TFIID binds to promoter
b) Binding of TFIIA and TFIIB stabilises the complex
c) RNA polymerase 11 and TFIIF bind, forming core pre-initiation complex
d) Binding of TFIIE and TFIIH completes the complex.
What happens to release RNA polymerase?
a)DNA is opened forming a transcription bubble (requires ATP hydrolysis by TFIIH)
B)Transcription starts, commonly pauses
C) Elongation factors join with RNA polymerase and initiation factors dissociate, gives elongation complex.
What is required for release of RNA polymerase in the last step?
Phosphorylation of elongated carboxy terminal domian of Polymerase II by TFIHH and positive transcription elongation factor-b.
